1. Field of the Invention
This invention relates to multi-purpose conductive gaskets for excluding electromagnetic interference (EMI), radio frequency interference (RFI) and also blocking environmental effects such as noise and moisture from the ambient atmosphere. More particularly, the invention concerns a conductive gasket or seal wherein a conductive sheath surrounding a resilient core is coated with a coating formed of a suspension of conductive particles. 2. Prior Art
Conductive seals and gaskets are known in a number of variations. Typically, the gasket bridges across metallic surfaces of adjacent or abutting conductive structures, for example at a closure of a cabinet, at a window or door, etc. The adjacent or abutting structures are to be continuously sealed to one another hermetically, to exclude air and moisture at least at low pressure differential, and also electrically, to exclude electromagnetic and radio frequency interference from affecting elements within the enclosure. Alternatively, the seal or gasket may be intended to prevent environmental or electromagnetic influences from escaping from an enclosure.
U.S. patent application Ser. No. 181,834, filed Apr. 15, 1988 (now U.S. Pat. No. 4,857,668) discloses a multi-function gasket of this general type. The gasket has a continuous, molded, resilient core, which is preferably fire-resistant foamed polyurethane. A flexible, electrically conductive sheath surrounds the foam core and is preferably bonded to a boundary layer around the outside of the core. The sheath can incorporate a metal mesh or foil, but preferably the sheath is resinous, for example Rip-Stop Nylon, and has a thin coating of metal plated thereon, for example silver. Alternative means are provided for mounting the gasket such that the sealed elements, such as electrically conductive doors, access panels and the like, are bridged across electrically and hermetically. Seals of this type are marketed by the Schlegel Corporation, Rochester, New York.
Seals and gaskets of the foregoing description might be appropriate for use in controlled environments where the temperature and humidity are maintained, and also in uncontrolled environments, where temperature and humidity vary. These seals are further employed in highly demanding applications such as marine use, wherein the seals are exposed to salt water, and even to space environments, wherein the seal and sealed elements are exposed to vacuum and high radiation.
Over the range of uses, there are important considerations which affect the choice of component materials for the sealed elements (hereinafter "flange plates") and for the gaskets or seals. Structural metals to be used in housings and cabinet structures, for example, may include various steels, possibly in the form of corrosion resistant alloys, aluminum, copper, magnesium, zinc and the like. It is also possible to alloy or coat metals such as steel to improve resistance to oxidation and corrosion. However, the use of dissimilar metals in conductive contact with one another inherently deteriorates at least one of the dissimilar metals when exposed to an electrolyte, due to galvanic action. When exposed to an electrolyte, (a salt solution having free ions) a migration of electrons occurs, which affects the dissimilar metals unequally due to their different electron valence conditions. The migration of electrons causes accelerated oxidation in the less noble one of the dissimilar metals. The conditions needed for galvanic corrosion are simply the dissimilar metals and the electrolyte in contact with them.
When a conductive gasket for excluding electromagnetic interference is placed between two metal flange plates, dissimilar metals are quite likely to be employed and must be placed in contact. The use of dissimilar metals is expected because desirable attributes in flange plates (e.g., strength and rigidity) are not the same as those in gaskets (e.g., flexibility and maximum electrical conductivity). It is also frequently the case that the area of the seal is exposed to an electrolyte. For cabinet closures, doors and other openable seals, condensation occurs at the junction between environments of different temperatures and humidities, separated by the seal. Corrosion can be expected.
Galvanic corrosion causes a physical deterioration of the seal resulting in deterioration of both electrical and environmental sealing performance. The sealed flange plates develop gaps relative to the gasket as a result of corrosion-induced unevenness of the flange plate surface. The extent of electrical connection across the seal deteriorates due to the fact that metal oxides as a rule are relatively less conductive than the corresponding elemental metal and produce increased electrical resistance between the flange plates or the like to be sealed. The increased electrical resistance reduces the seal effectiveness as does the physical displacement of the conductive parts.
The basic objective of shielding against electromagnetic interference requires blocking passage of electromagnetic waves at the seal or gasket. Both electric and magnetic fields are involved and interact. If an enclosure is surrounded by a theoretically perfect conductor, an incident electric field will be fully reflected because an electric field equal and opposite to the incident electric field is induced in the conductive enclosure. Magnetic fields induce a current in the conductive enclosure, and incident EMI or RFI waves include both electric and magnetic energy. The current induced in the enclosure by the field will vary across the thickness of the conductive enclosure, being greatest at the surface adjacent the incident field. A portion of the electromagnetic wave is reflected at the inner boundary of the enclosure, however, some of the radiation will reach the inside of the enclosure unless the conductive walls are very conductive and very thick.
A gap in the conductive shield produced either by physical spacing or by increased electrical resistance will allow a greater proportion of an incident electromagnetic field to radiate through the shield. Gasket material normally is flexible and has a lower conductivity than the material of the conductive enclosure or shield (i.e., a relatively higher electrical resistance). A highly conductive material is desirable to maintain a low resistance and minimal leakage at the interface between the gasket and the shield-defining flange plates or other elements.
If an air gap exists, the flow of induced current is diverted to those points or areas of the seal and sealed elements which are in contact. A high resistance joint, which may be caused by corrosion of the seal and sealed elements at their face and surfaces behaves in a manner similar to a gap.
In view of the corrosion caused by galvanic action between dissimilar metals, it may be desirable to employ similar metals for the sealed elements and the gasket. It is also appropriate to finish the metal which is expected to corrode, to decrease the incidence of corrosion. A relatively more corrosion-prone metal, for example, magnesium, tin, steel or aluminum, can be coated with relatively more-noble finishes which are less likely to corrode, for example, gold, platinum, silver, nickel, etc. Corrosion resistances thus improve for the overall sealed element. On the other hand, corrosion will be even greater if the seal breaks down or is applied so thinly as to be porous, because dissimilar metals are in contact at the site of an electrolyte. Some finishes, for example, chromate conversion coatings (e.g. Iridite for aluminum) are moderately conductive, and improve corrosion performance. Organic finishes can also be used. Such finishes are not fully effective because they are characterized by increased electrical resistance relative to bare metal, and a corresponding decrease in EMI shielding effectiveness; there is a possibility of corrosion should the finish become deteriorated; and in other respects leave room for improvement.
Conductive sheaths for seals available in the Schlegel product line employ various different metals, as needed for sealing between conductive elements of various descriptions. Most EMI/RFI gaskets are placed between two structural metal flanges, usually of aluminum or steel. A conductive gasket is interspersed, for example, the above-described Schlegel gasket with a conductive sheath of metallized nylon, on a foam core. It is also possible to use a sheath formed of a wire mesh or including metal fibers, for example including Monel or Ferrex. Monel is an alloy of nickel, copper and usually iron and manganese. Ferrex is a copper clad steel. While these materials are preferred for good conductivity, galvanic corrosion can be a serious problem.
Given the use of dissimilar metals, steps must be taken to control corrosion. Even when it is practical to seal flanges with finishes such that a more noble member is at the exposed surface and normally prevents electrolytes in the ambient atmosphere from reaching the less-noble metal, it is still desirable to design the interface properly to exclude moisture, and to cause the seal material to fill all gaps caused by uneven flange shapes, surface irregularities, bowing of the flange plates adjacent fasteners, and the like. Typically, designers of conductive elastomer gaskets will select a sheath material such as silver plated or aluminum filled elastomers, or silver-copper elastomers. The seals are kept clear of sump areas and/or are provided with drainage holes to remove any electrolyte accumulating due to condensation, etc. Dessicants may be employed, and protective paints are preferably applied to flange surfaces approaching the area of the seal. Military standards go so far as to prefer double seals, an external environmental seal being employed in addition to an electromagnetic seal. The outer, non-conductive environmental seal protects the inner, conductive seal from the ingress of electrolyte. (See e.g. Mil-Std-1250; Mil-Std-889; or Mil-Std-454).
According to the present invention, the incidence of corrosion can be minimized further, in a manner that also improves the durability of the gasket. The invention can be employed instead of, or in addition to a double seal structure. Contrary to the usual objective of placing only similar materials in direct abutment across the gasket interface, the invention relies on a further conductive coating on the gasket sheath. The further conductive coating is made from a dispersion of conductive particles in a preferably-curable elastomeric binder. The binder prevents direct exposure of the metal components of the seal to environmental electrolytes, reduces abrasion which could wear away protective or similar-metal coatings on the corrosion-prone flange plates, for example of aluminum, and provides the necessary conductive connection. The corrosion resistant and conductive coating, however, has the further and unexpected benefit of substantially decreasing the flammability of the seal. Preferably, the sheath coating is a colloidal dispersion of conductive particles, especially a colloidal dispersion of carbon particles in a urethane-based flexible elastomeric binder.
Whereas the prior art has recommended doubling seals to isolate an EMI seal from environmental influences, the invention allows the seal to inherently exclude environmental influences (especially electrolytes), while retaining good conductive characteristics.